Opencart. Почему mysql_insert_id не возвращается ID?

public function addCustomer($data) {
		if (isset($data['customer_group_id']) && is_array($this->config->get('config_customer_group_display')) && in_array($data['customer_group_id'], $this->config->get('config_customer_group_display'))) {
			$customer_group_id = $data['customer_group_id'];
		} else {
			$customer_group_id = $this->config->get('config_customer_group_id');
		}
		
		$this->load->model('account/customer_group');

		$customer_group_info = $this->model_account_customer_group->getCustomerGroup($customer_group_id);

		$this->db->query("INSERT INTO " . DB_PREFIX . "customer SET customer_group_id = '" . (int)$customer_group_id . "', store_id = '" . (int)$this->config->get('config_store_id') . "', language_id = '" . (int)$this->config->get('config_language_id') . "', firstname = '" . $this->db->escape($data['firstname']) . "', lastname = '" . $this->db->escape($data['lastname']) . "', email = '" . $this->db->escape($data['email']) . "', telephone = '" . $this->db->escape($data['telephone']) . "', custom_field = '" . $this->db->escape(isset($data['custom_field']['account']) ? json_encode($data['custom_field']['account']) : '') . "', salt = '" . $this->db->escape($salt = token(9)) . "', password = '" . $this->db->escape(sha1($salt . sha1($salt . sha1($data['password'])))) . "', newsletter = '" . (isset($data['newsletter']) ? (int)$data['newsletter'] : 0) . "', ip = '" . $this->db->escape($this->request->server['REMOTE_ADDR']) . "', status = '" . (int)!$customer_group_info['approval'] . "', date_added = NOW()");

		$customer_id = $this->db->getLastId();

		if ($customer_group_info['approval']) {
			$this->db->query("INSERT INTO `" . DB_PREFIX . "customer_approval` SET customer_id = '" . (int)$customer_id . "', type = 'customer', date_added = NOW()");
		}
		
		return $customer_id;
	}